Frequency modulation receiver tuning indicator



June 16, 1942. w, A HARRls 2,286,410

FREQUENCY MODULATION RECEIVER TUNING INDICATOR Filed April 9, 1941 2 Sheets-Sheet l ATTORNEY June 16, 1942. w. A. HARRIS FREQUENCY MODULATION RECEIVER TUNING INDICATOR Filed April 9, 1941 2 Sheets-Sheet 2 Ik foff/yewrfa INVENTOR -W .br/1,2 BY

ATTORNEY Patented June 16, 1942 FREQUENCY MODULATION RECEIVER TUNING INDICATOR William A. Harris, Nutley, N. J., assignor to Radio Corporation of America, a corporation of Dela- Ware Application April 9,

14 Claims.

My present invention relates to tuning indicator circuits for frequency, or phase, modulated carrier wave receiving systems, and more particularly to improved circuits of the type disclosed by John D. Reid in application Serial No. 362,910, filed October 26, 1940 and patented on February 3, 1942 as U. S. Patent No. 2,272,052.

In the last-mentioned application there has been disclosed, and claimed, a system for deriving two control potentials from the balanced detector circuit of a frequency modulation (designated FM hereinafter) receiver. These control potentials vary in the same sense, but at different rates, when tuning through the center frequency of a received modulated carrier wave. The potential difference of the control potentials is a maximum at resonance with the center frequency. The specific apparatus employed in that system required a special tube construction to make use of the aforesaid potential difference.

It is one of the important objects of my invention to improve the aforesaid Reid system, and to provide a simplified and effective means for obtaining visual tuning indication for FM4 receivers; accurate tuning of the receiver to the center frequency of a desired FM channel being indicated by minimum shadow angle on an electron-ray type of indicator tube.

Another important object, stated in terms of results sought to be secured, is to provide indicators free of false indications, possessing better sensitivity While using simple, standard tube types.

Still other objects of my invention are to improve generally the efficiency and reliability of FM receiver tuning indicators, and more especially to provide tuning indicators which are economically manufactured and assembled.

The novel features which I have believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. l shows a circuit embodying the invention;

Fig. 2 graphically depicts the operation of the system of Fig 1;

Fig. 3 shows a modification; and

1941, Serial N0. 387,571

(Cl. Z50-40) Fig. 4 illustrates another modification.

Referring now to the accompanying drawings, wherein like reference characters in the figures represent similar circuit elements, there is shown in Fig. 1 that portion of an FM receiver between the pre-limiter network and the audio amplifier. While those skilled in the art are very familiar at the present time with the construction of an FM receiver of the superheterodyne type, the following general description is given for the purpose of clarity. The FM band covers, at the present time, a range of 43 to 50 megacycles (designated mc., hereinafter). Each FM channel in the band has a width of 200 kilocycles (referred to as kc.). `'Ihat is, the carrier center frequency may be deviated a maximum of 100 kc. to each side. Any desired type of signal collector (such as a dipole, grounded antenna, loop or distribution line) may be used to feed the desired frequency modulated carrier waves to one or more tunable radio frequency amplifier stages. The amplified waves are then converted to an intermediate frequency (I. F.) by any of the well known types of converter networks. The center frequency (fc) of the converted waves is generally chosen from a range of 2 to 10 rnc., and, by way of example, let it be assumed that fc for the intermediate frequency waves is 4.3 mc.

The I. F. waves are amplified in one or more I. F. amplifiers, and then applied to a limiter stage. This is designated as I in Fig. 1. The rectangle schematically represents such a limiter, the curve within the rectangle showing the relation between input voltage and output voltage. As is well known the limiter acts in the manner of a saturated amplifier tube. It insures that solely frequency-Variable waves will reach the discriminator network of the second detector stage. Amplitude variation may be produced in the modulated carrier waves by noise impulses, fading and the prior resonant circuits. Since at the transmitter the modulation voltage amplitude appears in the carrier wave as a corresponding deviation of the carrier from the value fc, While the modulation frequencies themselves correspond to the rate of deviation of the carrier, it follows that no spurious amplitude variation can be permitted in the modulated carrier waves when applied to the detector.

The limited I. F. waves are now applied to the FM detector circuit. The latter may be of the type shown by S. W. Seeley in U. S. Patent No. 2,121,203, granted J'une 21, 1938. It is not believed necessary to describe the circuit in detail. In general, numeral 2 denotes the resonant primary circuit which is tuned to fc, and is in the plate circuit of the limiter tube. The resonant secondary circuit 3, reactively coupled to circuit 2, is tuned to fc. The high potential side of circuit 2 is connected to the mid-point of the coil of circuit 3 by the direct current blocking condenser 4. The tube 5 is a rectifier of the 6H6 type, and houses the diodes D1 and D2. The cathodes of the diodes are connected to opposite sides of the circuit 3. The anodes of the diodes are connected to the opposite ends of the series-connected load resistors R1 and R2. The anode end of resistor R2 is at ground potential. Each of the resistors is by-passed for I. F. currents, C1 shunting R1 and C2 shunting R1. The junction of R1 and R2 is connected to the midpoint of the coil of circuit 3 by choke coil 6.

Above the detection stage is shown the typical S-shaped characteristic curve of the circuit. The curve shows the relation between rectified output voltage (ordinates) and frequency swing from fc. At the center frequency of the applied waves the rectified voltage across R1 is exactly equal to that across R2, but the voltages are in polarity opposition. Hence, the total voltage across R1-R2 is zero at fc. As the carrier frequency swings from fc to either side there will be produced across the load R1-R2 a rectied voltage whose magnitude and polarity depend upon the extent and direction of carrier shift respectively. The modulation voltage component of rectified waves is tapped off by condenser 1, and the modulation voltage is fed to one or more audio amplifiers followed by a reproducer.

As shown in the characteristic curve of the `detector it is desirable that the applied I. F. waves have a center frequency equal to fc; that is, the operating I. F. Value. If the receiver be tuned so as to have the center frequency of the collected waves to one side of fc, then considerable and annoying distortion is had. A visual tuning indicator is employed to facilitate tuning so as to enable the set operator to adjust the station selector accurately. In the aforesaid Reid patent it has been shown how a variable shadow indicator tube of the fluorescent target type may be employed as a tuning indicator for an FM receiver, there being secured a minimum shadow angle for solely accurate tuning of the receiver. Previous attempts to use such indicator tubes were unsatisfactory by virtue of false minimum shadow angle for the case when no carrier Whatever was being received.

To operate the indicator tube T2, there is utilized the direct current voltage component of the rectified voltage across R1-R2. The tube T2 may be a 6U5 type tube, if desired. Such a tube has a triode direct current voltage amplier section, a fluorescent target electrode 8 and the electron ray control electrode 9 which is connected to the plate of the amplifier section. As those skilled in the art will know, both target and plate are supplied from the +B terminal of a direct current source. Resistor R7 reduces the voltage applied to the plate of the amplifier section. Hence, control electrode 9 will adjust the angular width of the electronic shadow produced on the target 8. When the electrode 9 is at a minimum potential difference relative to the target 8, then the shadow angle will be a minimum.

There will now be explained the manner of controlling tube T2. The anode end of R1 is connected to the anode of diode Ds by resistor R3. The cathode of D2 is connected to the control grid l0 of the amplifier section of tube T2. D3 can be a GHS tube or a 6J5 tube; in either case, the electrodes are connected to provide a single diode. The junction of R1 and R2 is connected to the grid of tube T1, say on-e of the 6J5 type, by resistor R4. Each of resistors Rs and 'R4 acts as an audio voltage lter, and they may be by-passed to ground by condensers C3 and C4 respectively each of low impedance to audio voltages.

The plate of tube T1 is connected to the positive end of Rv, while the cathode of tube T1 is connected, in common with the cathode of tube T2, to the ungrounded end of resistor Re. Resistors R5 and Rs are arranged in series between the cathode of diode D3 and the cathode of T1. The junction of R5 and Re is grounded. Merely by way of illustration, and in no way restrictive, the following constants are given:

R3; R4; R5; R1=l.5 megohms Rs=1000 ohms C1; C2250 micro-microfarads (mmf.) C2; C4=0.01 microfarad (mf.)

Considering, now, the operation of the system the voltage between the grid of tube T1 and ground is the direct current component of the rectified voltage across R2. The voltage between the grid IB of tube T2 and ground is a fraction of the total voltage across R1 and R2 when this voltage is positive. It is Zero when the total voltage is negative. The voltage at the cathodes of T1 and T2 is positive, because of the currents of tubes T1 and T2. The voltage increases when a positive voltage is applied to the grid of T1. The shadow angle of tube T2 depends on the difference between the voltages on the grid and cathode of this tube. The shadow angle will be a minimum when the grid IB is most negativ-e with respect to cathode. In such case the potential difference between electrodes 8 and 9 is a minimum.

In Fig. 2 there are plotted various curves to show the relation between frequency deviation of the carrier from fc and the rectified output voltages at `different points in the circuit. The Voltage across R2 varies with frequency in the manner shown by curve (a). The total voltage across resistors R1-R2 varies in the manner shown by curve (b). The voltage developed across resistor R5, and due to the action of diode D3 and resistors R3 and R5, is shown by curve (c). The curve (d) depicts the voltage at the cathode of the tuning indicator tube T2, and it will be noted that this curve .is rather similar in shape to curve (a). The voltage between the grid I and the cathode of tube T2 is shown by curve (e). Since grid I0 is always less positive than the cathode the voltage between grid i0 and cathode is always negative.

It will be observed that the maximum negative voltage on grid IU occurs at the frequency fc which gives zero voltage on curve (b). This is the desired and accurate tuning point.. In general, then, when a diode is used to obtain a voltage corresponding to one half of curve (b) (that is, diode D3) the difference between this voltage and the voltage of curve (a) is a maximum at the desired frequency. In the present 4circuit means 4is provided by which voltages proportional to curve (a) and the positive portion of curve (b) may be impressed on the cathode and grid respectively of a tuning indicator tube thereby obtaining the effect of the desired voltage difference.

In the circuit of Fig. 1 a voltage is developed across resistor Re even when no signal is being received. This voltage biases the triode section of tube T2, and thus causes the shadow on the target to remain partially closed. A wider opening can be obtained by applying a negative potential to that end of Rs which is shown connected to ground. When this is done there may be used an indicator tube of the 6E5 type, the latter being more sensitive to variations in voltage between grid and cathode. If a separate triode is used to operate the tuning indicator (as for example type 6SF5 with type 6AF6G) the target current of the tuning indicator does not have to pass through the resistor Rs. In such cases R6 may be increased thereby providing better sensitivity. Furthermore, the initial bias applied to the tuning indicator triode can be smaller. For such a circuit arrangement a double triode of the 6SC'7 type can be used to perform the functions of the triode T1 and the triode section of the tube T2.

In the arrangement of Fig. 1 the function of the tube T1 is to produce a voltage across the resistance Re which is proportional to the voltage across the relatively high resistance R2; the resistance Re having a relatively lower value. A frequency modulation detector circuit in which the voltage corresponding to curve (a) of Fig. 2 can be developed initially across a low resistance is shown in Fig. 3. The numeral 20 denotes a tube of the duplex diode-triode type. The diode anodes are strapped together, and function as a single diode anode. The diode anode is connected to one side of the tuned circuit 3, a direct current blocking condenser 3 being inserted in the connection. The control grid 2| of the triode section of tube 20 is connected to the opposite side of tuned circuit 3. The cathode 22 is connected to ground through resistor Rz, the b'y-pass condenser 23 being connected in shunt with the resistor R2. The second load resistor R1 is connected in series with Rz. It will be understood that resistors R1 and R2 of Fig. 3 correspond to the resistors R1 and Rz of Fig. l.

The grid and diode anode of tube 20 receive signals which have their maxima at different frequencies. The signal applied to the grid 2| causes a corresponding positive voltage to be developed across resistor R2, and the signal applied to the diode anode produces a negative voltage across resistor R1. The voltage across resistor R2 will vary with frequency in the manner shown by curve (a) of Fig. 2, and the voltage between the diode anode of tube 20 and ground will vary in the same manner as curve (b) of Fig. 2. In this way the desired output characteristic is secured. The connections to the tuning indicator tube T2 are also somewhat diiferent in the case of Fig. 3. Resistors R3 and diode D3 are arranged in series between the upper end of resistor R1 and grid I of the indicator tube. n The cathodes of the indicator tube are directly connected to the cathode end of resistor Rz, while grid I0 is connected to ground through resistor R5. In this circuit arrangement the resistor Re is not employed. The grid denser R2 and 23, the lower half of the input coil and choke 6 provide a detection circuit. Choke is grounded for this result.

In Fig. 4 a further modification is shown `wherein the upper triode section of tube 30,

2|, cathode 22, resistor-concondenser 1.

which may be a 6SC7 type tube, and the upper diode of tube 3l (which may be a 6H6 type tube) act conjointly as the frequency modulation detector. The other sections of these two tubes are the diode and the control tube for the tuning indicator circuit. Thus, the diode anode 40 is connected through condenser 4| to one side of the secondary circuit 3. The grid of the upper triode of tube 30 is connected to the low potential side of circuit 3. The load resistors R1 and R2 are in series between diode anode 40 and ground. The modulation voltage is taken off by The lower triode section of tube 30 acts as the direct current voltage amplifier for indicator tube 60. The latter is of the 6AF6G type, and has a pair of spaced shadows produced on independent targets 6| and 62. Each target has its own cathode and ray control electrode.

As in Fig. 3, the +B terminal of the current source is connected to the plate 5| associated with detector grid 50. The plate 5| is connected directly to the plate 52 by resistor R7. The grid 54 associated with plate 52 is connected to the diode D3 and R3. The grid 54 is connected to ground through R5. Condenser 55 by-passes R3 to ground. The cathodes of indicator tube 6l] are at ground potential, while cathode 4I of the detection diode is connected to the junction of R1 and R2. It will, therefore, be seen that the circuit arrangement of Fig. 4 is quite similar to that of Fig. 3.

Generically it will be seen that in each of the circuits the discriminator-rectifier operates the tuning indicator tube. The latter uses the combined output of the rectifiers with one of the individual outputs.

While I have indicated and described several systems for carrying my invention into eifect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but

that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a receiver of angular velocity-modulated carrier waves including a discriminator network, rectifier means connected with said network to provide a balanced output circuit and means for indicating resonance at the center frequency of said waves; the improvement comprising a rectiiier device connected between the balanced output circuit and indicating means to apply to the latter a variable control potential developed across the entire output circuit, and a direct current voltage connection to apply a portion of the said control potential to said indicating means in opposition to said entire circuit potential.

2. In combination with a source of frequencyvariable direct current voltage, an indicator tube having input electrodes, a connection including a rectier for applying said voltage between said input electrodes, and a direct current voltage connection between one of the input electrodes and a point on said source such that a portion of the said voltage is applied to said input electrodes in opposition to said first voltage.

3. In a frequency modulated carrier wave receiver, a detection network having an output circuit across which is produced a direct current voltage whose magnitude and polarity vary with the extent and sense of shift of the center fre quency of applied waves, a visual resonance indicator tube of the type having a fluorescent target and a control electrode for controlling the fluorescence on said target, said output circuit comprising a pair of series-connected resistors, an auxiliary electron discharge tube having input and output electrodes, said control electrode being connected to said output electrodes, a series path in shunt with said pair of resistors, said series path comprising a rectifier in series with a third resistor, said auxiliary discharge tube input electrodes being connected across said third resistor, and a direct current voltage connection between one of the said input electrodes and the junction of said pair of series resistors.

4. In a frequency modulated carrier Wave receiver, a detection network having an output circuit across which is produced voltage whose magnitude and polarity varies With the extent and sense of shift of the center frequency of applied Waves, an indicator tube of the type having a fluorescent target and an electrode for controlling the fluorescent on said target, said output circuit comprising a pair of series-connected resistors, an auxiliary tube having input and output electrodes, said control electrode being connected to sai-d output electrodes, a path in shunt with said pair of resistors, said path comprising a rectifier in series with a third resistor, said auxiliary tube input electrodes being connected across said third resistor, and a voltage connection between one of the said input electrodes and the junction of said pair of series resistors, the electrodes of said auxiliary tube being included in same tube envelope as said indicator tube electrodes.

5. In a frequency modulated carrier wave receiver, a detector having an output circuit across which is produced a direct current voltage Whose magnitude varies with the extent of shift of the center frequency of applied waves, a visual resonance indicator tube having an electrode for controlling the indications of said tube, said output circuit comprising a pair of seriesconnected resistors, an auxiliary tube having input and output electrodes, said control electrode being connected to said output electrodes, a path in shunt with said pair of resistors, said path comprising a rectier in series with a third resistor, said tube input electrodes being connected across said third resistor, a voltage connection between one of the said input electrodes and the junction of said pair of series resistors, and a common tube envelope housing the electrodes of the auxiliary tube and the electrodes f a rectifier of said detection network.

6. In combination with a frequency modulation detector of the type having a common input circuit, a pair of rectiers and a pair of seriesarranged resistors; a series path in shunt with said series resistors comprising a third rectier in series with a third resistor, an indicator tube having a control electrode, means for applying to said control electrode the voltage difference existing between the junction point of said series resistors and the junction of said third rectier and third resistor.

7. In a receiver of angular velocity-modulated carrier waves including a rectifier means connected to provide a balanced output circuit and means for indicating resonance at the center frequency of said waves; the improvement comprising a diode connected between the balanced output circuit and indicating means to apply to the latter a variable direct current control potential developed across the entire output circuit, and a direct current voltage path including an electron discharge tube to apply a portion of the said control potential to said indicating means in opposition to said entire circuit potential.

8. In combination with a source of frequencyvariable direct current voltage, an indicator tube having input electrodes, a direct current voltage path including a diode for applying said voltage between said input electrodes, and a second direct current voltage path including a tube between one of the input electrodes and a point on said source such that a portion of the said voltage is applied to said input electrodes in opposition to said first voltage.

9. In a frequency7 modulated carrier Wave receiver, a detection network having an output circuit across which is produced a direct current voltage whose magnitude and polarity vary with the extent and sense of shift of the center frequency of applied waves, a tube of the type having a fluorescent target and a control electrode for controlling the fluorescence on said target, said output circuit comprising a pair of series-connected resistors, an auxiliary electron discharge tube having input and output electrodes, said control electrode being connected to 'said output electrodes, a series path in shunt with said pair of resistors, said series path comprising a diode in series with a third resistor, said auxiliary discharge tube input electrodes being connected across said third resistor, and a direct current voltage connection including a tube between the junction of said pair of series resistors and said auxiliary tube.

l0. In a frequency modulated carrier wave receiver, a detection network having an output circuit across which is produced voltage Whose magnitude and polarity varies With the extent and sense of shift of the center frequency of applied waves, a tube of the type having a fluorescent target and an electrode for controlling the fluorescent on said target, said output circuit comprising a resistive path, an auxiliary tube having input and output electrodes, said control electrode being connected to said output electrodes, a second path, in shunt with said first path, comprising a rectier in series with a resistor, said auxiliary tube input electrodes being connected across said resistor, and a connection betweeen one of the said input electrodes and a point of said resistive path, the electrodes of said auxiliary tube being included in same tube envelope as said indicator tube electrodes.

1l. In a frequency modulated carrier wave receiver, a detector having an output circuit across which is produced a direct current voltage whose magnitude varies with the extent of shift of the center frequency of applied waves, an indicator tube having an electrode for controlling the indications of said tube, said output circuit comprising a resistive path, an auxiliary tube having input and output electrodes, said control electrode being connected to said output electrodes, a second path, in shunt with said first path, comprising a rectier in series with a resistor, said tube input electrodes being connected across said resistor, a voltage connection betweeen one of the said input electrodes and a point on said resistive path, and a common tube envelope housing the electrodes of the auxiliary tube and the electrodes of a rectifier of said detection network.

12. In combination with a detector of angular velocity-modulated carrier waves having a common input circuit, a pair of rectifiers and a resistive path; a second path in shunt with said resistive path comprising a third rectier in series with a resistor, an indicator tube having a control electrode, means for applying to said control electrode the voltage difference existing between a predetermined point of said rst path and the junction of said third rectier and resistor.

13. In a receiver of angular velocity-modulated carrier waves, a discriminator input circuit for converting frequency-variable waves into amplitude variable waves, a first tube embodying at least two electron sec-tions, a second tube embodying at least two diodes, means connecting one of said electron sections and one of said diodes to said input circuit to provide a rectier network for the said amplitude variable waves, a resistive path in circuit with said rectier network to provide a source of rectified voltage, an indicator device including a control electrode, means connecting the output electrodes of the second electron section of the rst tube to the said indicator control electrode, a second path comprising the second diode of the second tube, connected in shunt with said resistive path, and a control connection between said second diode and the input electrode of said second electron section.

14. In a receiving system of frequency modulated carrier waves, a network for transforming the frequency-variable carrier waves into amplitude-variable carrier waves, a rectifier network for the amplitude-variable waves comprising a tube having at least two electron sections, said sections being in circuit with said transforming network to provide rectified carrier voltage across a resistive path, a second path, in shunt with the rst path, comprising a device of unidirectional conductivity and a resistive impedance, an electronic indicator device provided with an electron control element, means, responsive to voltage variation across said resistive impedance, for controlling the potential of the control element.

WILLIAM A. HARRIS. 

